In the manufacturing of dynamic random access memories (DRAMs) the size of the memory cell is the main contributing factor to the density and overall size of the device. A manufacturer of DRAMs has motivation to increase the storage capability, while maintaining the smallest die size possible, as the smaller die size results in a lower cost per device. As mentioned, the main contributor to the size of a memory device is the amount of space required for each storage cell that makes up the storage array. In that regard, DRAM fabrication engineers have focused on structures, on materials to make the structures and on methods to fabricate the structures necessary to make a storage cell.
To save space, the capacitor of the storage cell must reduce in size and yet maintain adequate capacitance to retain a sufficient charge during DRAM operation. There are several approaches to the capacitor design, for example trench capacitors formed in the substrate of a wafer or a stacked capacitor formed above the wafer substrate, to name two. Regardless of the design chosen, the size of the capacitor must be reduced and yet maintain sufficient capacitance as mentioned previously. Two of the main contributors to capacitance are the surface area of the capacitor plates and the dielectric quality of the insulator separating the capacitor plates. Major engineering efforts have gone into both areas.
In regards to dielectric quality, thin film dielectrics having high dielectric constant characteristics have emerged as the dielectric of choice, as the thinnest film that can be placed between the capacitor plates to prevent dielectric breakdown when a charge is present on the capacitor plates, drastically increases capacitance. With increased capacitance, the overall size of the capacitor can be reduced. However, thin film dielectrics present some challenges in fabricating the complete storage cell structure, which includes a storage cell access transistor and a storage capacitor.
One main challenge and a critical area of concern is oxidation punch through, which is important to avoid when forming thin film dielectrics. Oxidation punch through refers to the mechanism of atomic oxygen diffusing completely through a dielectric film. In the case of a capacitor cell dielectric, if oxidation punch through was allowed to occur a portion of an underlying diffusion region of an access transistor would become oxidized and thus diminish the transistor's operating characteristics. It is critical that oxidation punch through be at least reduced or ideally avoided altogether. When dealing with thin film dielectrics the dielectric film needs to be thick enough to sufficiently to reduce oxidation punch through. The minimum thickness of the dielectric film is dependent on the required oxidation time and temperature used and is particularly critical to maintain when using the dielectric film as a capacitor cell dielectric. It is also important that the dielectric film be a uniform film in order to minimize the overall thickness of the film.
One of the thin dielectric films of choice is nitride (i.e., silicon nitride) as nitride possesses sufficient dielectric constant characteristics and can be deposited as a very thin layer (less than 100 .ANG.). However, a nitride film of this thickness is difficult to deposit uniformly on a surface that is made up of different types of material, especially materials that are not receptive to nitride deposition. When trying to deposit thin nitride films on different types of materials, the surface free energy involved in the deposition reaction is different for each of the different types of materials. Thus the different types of materials do not allow the formation of a uniform dielectric film, particularly layers less than 100 .ANG..
The present invention teaches a method to successfully form uniform dielectric films as will become apparent to those skilled in the art from the following disclosure.